1. Field of the Invention
This invention relates to a cyclic process for the removal of carbon dioxide and water contaminants from a feed gas stream by adsorption on molecular sieve adsorbents. More particularly, the invention relates to an improved method for the regeneration of the adsorbents. Removal of water and carbon dioxide contaminants from feed gas streams is often necessary prior to further processing for the feed gas stream. Examples of such further processing requiring removal of water and carbon dioxide contaminants is the cryogenic distillation of air, the purification of hydrogen gas streams, hydrogen and carbon monoxide streams, or the cryogenic liquefaction of natural gas feed streams to produce liquefied natural gas (LNG).
2. The Prior Art
Crystalline zeolites, commonly referred to as molecular sieves, have been used for the adsorption of water and/or carbon dioxide from feed gas streams in the past. Various processes involving the adsorption and regeneration phases for such molecular sieves have been used. Because of the great affinity of the molecular sieves for water and carbon dioxide molecules which makes possible their removal from feed gas streams to very low concentrations, substantial time and energy must be expended in the regeneration of the molecular sieves after they have reached saturation during the adsorption phase. The heat, time and energy required to regenerate the molecular sieves in the adsorbent beds thus determined to a large extent the size of the equipment and the total energy requirements. As a consequence, considerable effort has been devoted in the past to the development of efficient regeneration methods.
Various methods have been employed for the regeneration of the adsorbent to effect the desorption of the impurities. These methods have included the depressurization, heating, and the elution of the adsorbent beds and combinations of the three. Because of the strong affinity of the molecular sieve adsorbents for carbon dioxide and water impurities and the high adsorptive capacity, the most commonly employed regeneration process has involved depressurization of the adsorbent beds, the heating of the beds to elevated temperatures and the purging of the adsorbent beds with the regeneration gas so that it is at least partially freed of the impurities. Often the adsorbent beds are heated by means of the regeneration gas itself, the regeneration gas having first been heated before introduction to the adsorbent beds. Due to the large adsorbent and pressure vessel masses which must be heated and subsequently cooled in commercial scale facilities, substantial amounts of energy are expended for both heating and cooling and, consequently, cycle times required to achieve the requisite heat transfer are correspondingly long.
It has been recognized that due to the relative affinities and capacities of molecular sieves for water and carbon dioxide, that the water is preferentially adsorbed in a first adsorbent zone proximate the introduction point of the feed gas stream to the molecular sieve adsorbent. The carbon dioxide is secondarily adsorbed by the molecular sieve in a second adsorbent zone downstream from the water zone. Because of the greater affinity of the molecular sieve for water than carbon dioxide, more severe regeneration conditions are required to desorb the water than the carbon dioxide. This generally involves a higher regeneration temperature.
In order to reduce the heating and cooling requirements and the quantities of purified regeneration gas, and to shorten the cycle times, various methods have been devised for regenerating the carbon dioxide and water zones under differentiated conditions. For example, see U.S. Pat. Nos. 3,150,942; 3,710,547; 3,738,084; and 3,841,058. French Pat. No. 2.005.910, also discloses method and apparatus for the adsorption of water and carbon dioxide from gas mixtures and a method for the regeneration of the molecular sieves used for the adsorption.
Accordingly, it is an object of this invention to provide an improved process for the adsorption of water and carbon dioxide contaminants from a feed gas stream by adsorption on molecular sieve adsorbents and subsequent regeneration of the adsorbent. It is further an object of the invention to provide an improved process that consumes less energy for the heating and cooling cycles during adsorbent bed regeneration.
It is still another object of the invention to reduce the regeneration cycle time in consequence of the reduced heating and cooling requirements and to correspondingly reduce the adsorbent bed volumes and to reduce the operating and capital costs of the process and apparatus.